Solders are typically used in the microelectronics industry to join microelectronic circuit elements and packages (die, wafer, chip, etc) to each other and/or onto a circuit board. A solder may be present as a paste, as a metallization, metal layer or metal coating onto one or more joining surfaces, or as preforms placed upon the surfaces to be joined. These metalized surfaces or preforms may be used in a fluxless process such as thermo-compression bonding, or in conjunction with a chemical flux to promote melting, wetting, and metallurgical interaction between the surfaces to be joined. Depositing metallic layers or coatings is a common approach used during wafer level packaging (WLP) such as Au—Au and Cu—Cu thermo-compression bonding. It is also used in the hermetic sealing of microelectromechanical systems (MEMS), and for the joining of laser diodes to heat sinks in optoelectronic applications.
Conventional soldering involves the use of solder pastes, which are mixtures of metal powders and various chemical additives to improve the rheological, chemical, and mechanical characteristics of the pastes. The metal powders present in conventional solder pastes are of a uniform composition, either a pure metal or metal alloy (mixture of metallic elements). The melting point of conventional solder pastes is defined by the melting point of the metal powders contained therein. When these conventional solder pastes are heated to a soldering temperature above their corresponding melting point (or liquidus if not a eutectic), they become fully liquid, wet the surfaces/components to be joined, and are subsequently cooled to solidify as a uniform metallurgical joint. Upon reheating, the solder joint will exhibit the same (or similar) melting behavior as during the initial soldering step.
To comply with the Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, the microelectronics industry transformed from using a conventional Pb—Sn eutectic solder, to any number of Pb-free solders. The primary Pb-free solders adopted by the microelectronics industry have melting temperatures that are higher than the conventional Pb—Sn eutectic solders, and as a result require higher furnace and reflow temperatures during the soldering process. These increased temperatures have an adverse effect on the microelectronic components and printed circuit boards (PCB), and can cause chip failure and/or PCB warpage.
There are several RoHS compliant conventional solders that have melting temperatures below that of the conventional Pb—Sn solders, however the melting temperature of these conventional solders are too low for safe operation in many standard service environments. It is therefore desirable to have a solder that is capable of being processed at lower melting temperatures relative to the currently adopted Pb-free solutions, yet retain the thermal reliability during service/operation that would be displayed by a higher melting temperature solder. Since conventional solders cannot meet both of these requirements, a non-conventional solder is required.
Some prior art seeks to use a low-melting point and high-melting point material within a solder, to achieve both the low-temperature joining requirements and high temperature reliability as outlined above. U.S. Pat. Nos. 4,740,252, 5,328,521, 5,928,404, and 7,017,795 describe composite solders that comprise a low-melting material that, when heated beyond its melting point to a soldering temperature, wets both the joining surfaces and high-melting particles dispersed therein. Subsequent cooling solidifies the low-melting material and forms a composite solder joint that consists of the high-melting particles dispersed throughout.
U.S. Pat. No. 5,540,379 discloses a soldering process that utilizes two separate solder powders with differing melting points, said powders being heated in a two-stage reflow process. During heating above the low-melting solder powder (a tin-lead-bismuth alloy), a substantially flat solder surface is formed on the solderable portions of a PCB. This layer of solder consists of an aggregate-like structure of the unreflowed high temperature solder powder (a tin-lead-silver alloy) distributed within a matrix of the reflowed or melted low temperature solder powder. Components can then be placed upon said solder surface, and the assembly is heated to a reflow temperature above that of the high melting solder powder to form a solder fillet between the solderable portions of the component and the PCB.
Prior art exists that seeks to use solder powders of different types having different melting points. U.S. Pat. No. 5,382,300 discloses a solder paste that uses two separate types of solders that have different melting points, one of which is a eutectic or near-eutectic Pb—Sn alloy powder, the other component comprised of an elemental or alloy powder. The soldering temperature is chosen so that both solders liquefy, however the differing melting points of the two solders ensure that the solder paste does not melt homogeneously.
U.S. Pat. No. 5,803,340 discloses a solder paste composition that uses two separate solder powders with differing melting points. This solder paste acts as a composite solder during a flip chip bumping process. The low-melting solder employed is chosen such that it melts at or below a bumping reflow temperature, and this temperature does not degrade a photoresist mask. Upon removing the photoresist mask, the chip is reheated to a chip mount reflow temperature that is higher than the bumping reflow temperature, and sufficient to melt both the low and high melting solders. This yields a homogeneous alloy possessing a Pb content between that of the original solder powders, and higher solidus and liquidus than the alloy powder of the low-melting solder.
U.S. Pat. No. 5,376,403 discloses conductive adhesive compositions which include a solder powder and high-melting particles dispersed throughout a conductive ink, wherein said compositions are ideally suited for creating the conductive paths on printed circuits. The high-melting particles are added to the conductive adhesive to improve electrical conductivity over conductive polymer thick films. The connection of these high-melting particles by the low-melting material further improves the electrical conductivity of the pastes. Through dissolution of the high-melting particles into the low-melting liquid, the composition of the low-melting material changes increasing its melting temperature. This causes the conductive ink to resist melting at common soldering temperatures.
U.S. Pat. No. 5,389,160 discloses a solder paste, comprising a mixture of a first metal powder and a second metal powder dispersed in an expendable vehicle, said first metal powder composed of an alloy of tin and bismuth having a melting temperature, said second metal powder chosen from the group of gold and silver and present in an amount sufficient to dissolve into the liquid of said first metal powder to increase the melting temperature thereof. U.S. Pat. No. 5,429,292 discloses a method for forming a solder connection using this solder paste.
U.S. Pat. No. 5,229,070 discloses a solder composition wherein a first metal powder containing predominantly tin having a first melting temperature, and a second solder powder being a tin alloy containing elements from the group consisting of indium and bismuth, and having a second melting temperature less than the first melting temperature. This solder composition is heated to a temperature above the second melting temperature to form a liquid that initiates wetting of a faying surface, and is subsequently heated to a higher temperature to promote complete liquefaction of the first metal powder. The fully liquid solder is then cooled to form a joint that possesses a solidus temperature above the liquidus temperature of the second solder powder.
U.S. Pat. No. 6,613,123 discloses solder compositions which display a variable melting point. Said solder compositions comprise a metal or metal alloy powder having a low melting point with a metal powder having a higher melting point. Upon heating to a process temperature, in-situ alloying occurs between the low and high melting point powders such that solidification occurs at the solder temperature with little or no intermetallic phase formation, thus creating a new higher solidus (or melting) temperature.
U.S. Pat. No. 4,834,794 discloses a solder composition comprising a solder powder selected from the group consisting of Sn and Pb, containing a metal additive selected from the group consisting of Bi, In, and Cd, to depress the melting point of said solder powder, mixed with an alloy powder formed from elements selected from the group consisting of Se, Te, and Tl, and elements from the group consisting of Sn and Pb. Upon heating to a soldering temperature of at most 130° C., said alloy powder reacts with said additive metal to cause the soldering layer to have a remelting temperature higher than a melting point of the solder composition or solder powder.